A secretion is any substance produced and released by a gland or specialized cell in the horse’s body to serve a physiological function. Unlike excretion, which removes metabolic waste, secretion delivers a useful product — an enzyme, hormone, lubricant, or protective fluid — to a target site, either locally or via the bloodstream.
The horse secretes across every major system, and several of these secretions are clinically important. The salivary glands produce roughly 10 to 12 liters of saliva per day during active grazing; this secretion is alkaline, buffering the acid load arriving from the stomach and beginning starch digestion. Disruption of salivary secretion, as occurs with choke or botulinum toxin, creates measurable downstream acid injury. The gastric mucosa secretes hydrochloric acid and pepsinogen continuously; the horse’s stomach does not stop secreting acid between meals, which is why stall confinement without constant forage access drives squamous gastric ulcer disease — the acid secretion continues against an unprotected mucosa. The pancreas secretes lipase, protease, and amylase into the duodenum; inadequate pancreatic secretion (rare but documented in cases of chronic pancreatitis) impairs fat and protein digestion and produces weight loss that does not respond to increased caloric intake.
Among endocrine secretions, the adrenal cortex secretes cortisol; chronically elevated cortisol secretion — as in pituitary pars intermedia dysfunction (PPID, equine Cushing’s disease) — drives the laminitis, muscle wasting, and immune suppression that define that condition. The thyroid secretes thyroxine; hypothyroid secretory insufficiency in mares has been associated with poor reproductive cycling, though frank clinical hypothyroidism is less common in horses than sometimes claimed.
Two secretions distinctive to the horse deserve specific mention. The chestnut — the horny, keratinized growth on the inner aspect of each leg — is a vestigial structure whose surface is produced by a slow sebaceous secretion; chestnuts grow continuously and are trimmed as routine hoof care. The sweat gland secretion of the horse is unique among mammals in that it is a protein-rich, latherin-containing fluid; latherin is a surfactant that allows sweat to penetrate the dense coat and spread across the skin surface for evaporative cooling, which is why horse sweat foams under tack — the lather is the surfactant at work, not a sign of distress.